Vehicles know from the practice are designed for increasing the traveling comfort with so-called driving speed control units, by way of which a specific speed value of the vehicle speed during a driving operation of the vehicle can be automatically adjusted without the driver actuating an accelerator pedal and can also be automatically kept in case of changed operation conditions of the vehicle. At the same time, it is provided in the known systems to obtain the adjusted speed value or the requested target speed of the vehicle by varying an input torque of the prime mover, by an adequate command to a motor control and control of a brake control of a brake system of the vehicle.
It is further provided that in case of a driver's actuation of the brake pedal or accelerator pedal the vehicle speed control is terminated, the driver being able to reactivate the vehicle speed control via an adequate command device—such as by pressing a button and automatically to readjust the value of the speed adjusted prior to the actuation of the brake pedal or the accelerator pedal via the vehicle speed control.
There have been additionally developed for further improvement of the traveling comfort, so-called adaptive vehicle speed controls by way of which a distance control, together with an automatic speed control, can be carried out. Such adaptive vehicle speed controls control the system of the vehicle so that the vehicle is automatically decelerated when, for example, the vehicle drops below a certain distance from an object or a requested target driving speed is exceeded during a descent of the vehicle. If after engagement of the adaptive traveling speed control, it is detected that the requested distance value has again been exceeded or the requested target speed of the vehicle again has been reached, the originally requested operating condition of the vehicle is reproduced, it being possible under certain circumstances to accelerate the vehicle by adequate control of the prime mover to the requested target speed.
To support a road speed control or an adaptive road speed control, when downhill travel of a vehicle is detected, special programs with special shifting characteristic lines, coordinated with downhill travel, are started so as to adjust a brake or thrust torque on the output of the drivetrain of the vehicle such that the braking system of the vehicle can be unloaded during long descents.
DE 28 52 195 A1 has disclosed a control device for an automatically shifting transmission by way of which a driving strategy is assisted during mountain travel by a mountain detection. The mountain detection is also used during an activated speed control or in the course of a cruise control transmission in order to control, during an ascent, for example, the transmission by shift characteristic lines which request an upshift only when the rotational speeds of the prime mover are higher than in flat country and are set by way of the greater hysteresis between upshift and downshift characteristic lines.
From DE 40 37 248 A1 has become known for assistance of a vehicle speed control to release, in the transmission, at least one downshift in direction of a lower transmission gear and/or trigger the closing of a converter clutch during an ascent so as to select the gear of a transmission adequate at the moment for the speed control.
However, the last two mentioned procedures known from the prior art have the disadvantage that the methods can be coordinated only to an insufficient extent with different gradient values and changed vehicle loads wherefore they do not assist a traveling speed control to the extent needed for greater traveling comfort.
This means that known descent detections during downhill travel of a vehicle with simultaneously active travel speed control or simultaneously active adaptive travel speed control do not make available the functionality desired inasmuch as the varying states of the vehicle, due to changing loads and to the constantly varying travel profiles, cannot be covered via application of the shifting characteristic lines of a transmission and a driver is often compelled when operating a vehicle adequately to counteract divergences from the required target speed by manual engagements such as by actuating an accelerator pedal or brake pedal.
Should the driver wish to prevent the exit from the traveling speed control, there remains only the manual engagement via limiting positions of a change to a tip driving program, the driver having to react to changes of the downhill gradient either with downshift or upshift requests so as to be able to adjust the requested target speed of the vehicle.
Therefore, the problem on which this invention is based is to make available a method for the control of a drivetrain of a vehicle with one prime mover and one transmission by way of which a target speed of the vehicle, under active drive speed control, can be adjusted or kept during an ascent without manual engagements by the driver even under changing operating conditions.